NASA Contractor Report 177616

The Lift-Fan Powered-Lift AircraftConcept: Lessons LearnedWallace H, Deckert

(NASA-CR-177616) THE LIFT-FAN

POWERED-LIFT AIRCRAFT CONCEPT:

LESSONS LEARNED (NASA) 79 p

N94-15718

Unclas

G3/05 0190802

CONTRACT A25364D

September 1993

NASANational Aeronautics andSpace Administration

https://ntrs.nasa.gov/search.jsp?R=19940011245 2020-06-26T02:48:50+00:00Z

NASA Contractor Report 177616

The Lift-Fan Powered-Lift AircraftConcept: Lessons LearnedWallace H. Deckert

Retired NASA Ames Researcher

Oceanside, CA

Prepared forAmes Research CenterCONTRACT A25364D

September 1993

N/ ANational Aeronautics andSpace Administration

Ames Research CenterMoffett Field, California94035-1000

TABLE OF CONTENTS

Introduction ................................................................................................................................. 1

Mission Applications ............................................................................................... -.................. 2

Lift-Fan Aircaft Design Studies .................................................................................................. 5

Design Integration ....................................................................................................................... 23

The Avrocar Flight Evaluations .................................................................................................. 28

Concluding Remarks ................................................................................................................... 31

Bibliography ................................................................................................................................ 32

Figures ......................................................................................................................................... 35

Appendix I: Acronyms for Lift-Fan Aircraft .......................................................................... I-1

Appendix II: Chronology of Lift-Fan Aircraft Design Studies ................................................ II-1

iii PRECEDCl_ PA_E BLANK NOT FtI.MED

_ I kl _ '_ _i ....

Introduction

This is one of a series of papers presented at a

Seminar held at NASA Ames Research Center, 1992.

The stated objective of this effort is to conduct a

thorough review of, and document, the lessons learned from

past research related to lift fans and lift-fan aircraft

concepts. This includes conceptual design studies, wind

tunnel tests, piloted simulations, flight tests of aircraft

such as the XV-5B, and propulsion system component tests.

The results will be made available to appropriate government

personnel to help guide enabling technology validation

programs.

A team of experts has documented lessons learned from

past lift-fan aircraft research. These experts are NASA

Ames retirees, and staff from NASA Ames and NASA LeRC. Each

is the author or co-author of a technical paper in this

series. The hope is that this effort will help foster the

continued advancement of lift-fan aircraft technology, and

in particular, "without reinventing the wheel".

The first lift-fan aircraft experimental research

investigation in the nation featured a "lift fan" in a two-

dimensional wing. Lacking a real lift fan, a propeller was

used to simulate a lift fan. This research, conducted by

Mr. David H. Hickey of NACA Ames, was initiated in 1956,

thirty-six years ago. In 1957 Mr. Hickey published NACA RM

A57F03, "Preliminary Investigation of the Characteristics of

a Two-Dimensional Wing and Propeller With the Propeller

Plane of Rotation in the Wing-Chord Plane". Since then,

NACA and NASA researchers and contractors have authored

hundreds of technical publications on the advancement of

lift-fan aircraft technology. The summary report in this

series contains a Master Bibliography of a selected set of

these publications.

Most lift-fan aircraft research was conducted in the

context of being applicable to subsonic aircraft. For

supersonic lift-fan aircraft, takeoff, landing, conversion

to and from powered-lift flight, and some mission legs are

at subsonic speeds. This fact, and because some research was

generic, makes some subsonic research applicable to

supersonic aircraft. Some research was specific to

supersonic aircraft.

Lift-fan aircraft research was applicable to all

categories of powered-lift aircraft including those known by

the acronyms STOL, VTOL, V/STOL, and STOVL. See Appendix I

for definition of, and aircraft design implications for, the

various powered-lift aircraft acronyms.

Mission Applications

Lift-fan aircraft are competitive throughout thepowered-lift spectrum; STOL, VTOL, V/STOL, and STOVL. Theyare applicable to supersonic and subsonic aircraft, to civiland military aircraft, to fighters and transports, and topersonal aircraft.

The applicability of lift-fan aircraft is partlybecause vertical flight often requires dynamic verticalflight as opposed to sustained steady-state hovering flight

while in the vertical flight mode. Lift-fan aircraft are

competitive for certain missions that do require sustained

hovering flight as illustrated in two examples that follow.

One example is for the class of missions in which time

is of the essence and radius of action is long, such as

ocean-wide search and rescue. The helicopter, with its

hovering capability, is not competitive for these missions

that do require hovering flight because of limited range.

Tilt rotor aircraft may have the range, but the higher speed

lift-fan aircraft have the advantage when time is of the

essence.

Another example is for the class of missions in which

the sustained hovering requirement is for a short period of

time, such as inflight vertical delivery of supplies for

civil national disasters, and for replenishment and other

military missions.

Though promising for certain missions that require

sustained hovering flight, lift-fan aircraft are most

promising for the civil and military missions that require

dynamic vertical flight.

Aircraft design and operational considerations differ

for sustained hovering flight and dynamic vertical flight.

The considerations include fuel usage, reingestion, FOD,

visibility, perceived noise, nonproductive time, ground-

effect-induced performance changes and attitude upsets and

instabilities, detectability, and requirements for

preparation of the terminal site.

A lesson learned was that differences favor dynamic

vertical flight.

For example, a ground-effect-induced upsetting moment

during hovering flight may not be detectable during dynamic

takeoff or landing. For lift-fan aircraft on missions

requiring dynamic vertical flight, fuel usage during takeoff

or landing and those problems associated with steady-state

sustained hovering flight may not be issues.

Though lift-fan aircraft technology can be utilized to

2

meet todays missions, it is better characterized as a

technology for missions that yield new civil opportunities

and new military strategies.

Civil opportunities include new or expanded services

in such areas as:

I. Ocean resource operations, with "terminals" on oil

rigs, ships, and mineral exploration platforms.

2. Direct city-center to city-center transportation.

3. Direct corporate office to factory service.

4. Transportation for underdeveloped countries.

5. Transportation for inaccessible communities.

6. Search and rescue.

7. Emergency medical services.

8. Disaster relief.

9. Private flying, including to/from terminal sites

that are not airports.

Military strategies include new or expanded modes of

operation in such areas as:

i. Enhanced operations from aircraft carriers.

2. Operations from "nonaviation" ships.

3. Operations from civil ships in time of need.

4. Solution for total runway denial.

5. Use of austere dispersed land-based sites.

6. Search and rescue.

7. In-flight vertical delivery.

8. Counter for terrorist activity

A view that is too limited is that lift-fan aircraft

are promising for new civil opportunities and military

strategies because of takeoff and landing capability such as

STOVL or V/STOL.

A lesson 16arned was that lift-fan aircraft are

promising for several reasons, namely as follows:

I. Short and/or vertical takeoff and landing.

2. Near-terminal departure and approach, and up-and-away flight performance and maneuverability. Enhancementsare due to in-flight thrust vectoring, low-speed attitudecontrol systems, and more.

3. Aircraft design tradeoffs. For example, lift-fan-in-fuselage installations compromise fuselage design to adegree. However, unlike for conventional aircraft, thelift fan assists takeoff and landing to the degree that thewing can be optimally designed for cruising flight.

4. Advantages from use of ground-based facilities.

For example, lift-fan aircraft are compatible with ski-

jumps. A STOVL aircraft "can not be thrown into the air

before it is ready to fly" because minimum control airspeed,

Vmc, is zero and thrust-to-weight ratio is not limiting.

Civil lift-fan aircraft can utilize existing ground

facilities in new ways, such as departing within the

boundaries of the airport to eliminate noise annoyance in

surrounding suburbs.

5. Total system considerations. This requires no

explanation to DOD who are experts at total weapons system

analysis. If the aircraft carrier does not have to be

turned into the wind in order to launch some of its

aircraft, DOD understands and accounts for that advantage.

On the other hand, to the author's knowledge, there are no

civil authorities responsible for total transportation

systems. If a higher airline ticket price enables lower

ground transportation costs, saves time, and lowers taxes,

the airline operator is not impressed that his ticket prices

are higher than competition. Despite this reality, there

are significant total transportation system gains from use

of civil lift-fan aircraft.

It is not unusual for military and civil potential

customers to be as interested, even more interested, in all

of the above attributes of lift-fan aircraft as they are in

the one well-known attribute concerning short and/or

vertical takeoff and landing. Elaboration is found in the

various papers presented at the Seminar.

4

Lift-Fan Aircraft Design Studies

This section contains a description of and the lessons

learned from lift-fan aircraft design studies. The studies

are those about many powered-lift aircraft concepts of which

the lift-fan aircraft concept was one, and those that were

exclusively about lift-fan aircraft concepts.

The section is organized into eight subsections, with

each subsection covering an aircraft design study or a re-

lated set of studies. The title for each of the subsections

is the title used in the final report(s). Also see Appendix

II which presents these design studies in a brief chronolog-

ical format, and further correlates the presentation with

the Bibliography.

The time period is 1956 to 1992. During the period

1956 to 1962, there were no NACA/NASA lift-fan aircraft

design studies per se. Rather this period included explor-

atory lift-fan aircraft research, support for the Avrocar

and XV-5A lift-fan aircraft, and NASA/General Electric

studies of lift-fan propulsion. The section begins with the

first NASA aircraft design study that included lift-fan

aircraft, a study published in 1964.

Design and Operating Considerations of Commercial STOL

Transports

This was a NASA in-house aircraft design study that

supported FAA's program for a new short-haul aircraft for

the local service airlines. Payload included 20 passengers

and range was 690 sm that enabled four i00 sm stage lengths

without refueling. Though emphasis was on propeller STOL

aircraft, the study did include 1 lift-fan STOL. Also

included were propeller VTOL and CTOLs and a turbofan CTOL

for comparisons with the STOLs.

Using the original figures drawn 30 years ago, the

final report argued: figure 1 -- that local service airlines

approach at 90 kts, require airfields 3500 ft long, and thus

can fly into 35% of existing fields, whereas the lift-fan

STOL with a 60 kt approach could land in 95% of the existing

fields; figure 2 -- that approach and takeoff patterns are

much less for STOLs than CTOLs, and this has important

implications in reducing traffic control problems, time lost

in air maneuvering and more; and figure 3 -- that STOLs can

land safely under IFR conditions of lower ceilings and less

visibility which also improves schedule reliability. After

30 years neither the figures nor the text require change.

Some particulars about the lift-fan STOL were: fan-in-

wing with 2 interconnected gas generators and 2 tip-turbine

5

lift fans of 5.4 ft dia and 8200 ib thrust each; and an

aircraft of 25800 Ib DGW with a cruise airspeed of 390 kt

and an approach airspeed of 60 kt into a 1500 ft field. See

no figure for a drawing of this lift-fan aircraft, because

no one ever drew it.

One study result was figure 4. It says the DOC of the

jet having a 60 kt approach speed (i.e. the lift-fan STOL)

is not much more than that of the jet CTOL, and further as

stage length reaches 300 sm or more the lift-fan aircraft

starts becoming competitive to propeller aircraft. The

author can not resist adding to this paragraph that at the

time we were concerned about fairness and objectivity so for

the DOC computation we raised the price of gas to the high

level of 12.5 cents/gallon.

One lesson learned from this first design effort was

the knowledge that can be gained by including reference

aircraft on each side of the powered-lift spectrum compared

to the aircraft under study. For example, if design is

about STOVL, then the scope should include designs to the

same mission (as much as possible) of one reference V/STOL

and one reference STOL.

6

Study on the Feasibility of V/STOL Concepts for Short Haul

Transport Aircraft

These were the first NASA contractual studies that

included design of lift-fan powered-lift aircraft. Prior to

these NASA studies, U.S. Air Force contractual studies of

large military transport designs had been completed. Known

as the CX-6 studies, the Air Force studies included design

of lift-fan VTOL transports, and these CX-6 designs were

used as reference points for initiation of some of the NASA

contractual designs.

The NASA studies, conducted during the mid 1960s,

included NASA in-house activities and contracts to Boeing,

Vought, and Lockheed. Prior to go-ahead, NASA spent months

establishing the rules and creating a comprehensive document

on design goals and criteria.

A lesson learned was the importance, for directing

studies, for obtaining meaningful results, and for

efficiency, of preparations prior to initiation of aircraft

design studies.

Aircraft design goals included 500 statute mile range,

cruise airspeed near miminum direct operating cost, 60 and

120 passengers, reserve fuel and revenue cargo, 1970

propulsion, aircraft low-speed control criteria for all

engines and for critical engine inoperative, and controlcriteria that varied as a function of aircraft design gross

weight. Studied were five VTOL concepts with trimmed

thrust/weight ratios of 1.15 all engines and 1.05 critical

engine inoperative, and four STOL concepts for commercial

field lengths of i000 and 2000 feet that corresponded to 55

and 85 knot approach airspeeds.

The VTOL concepts, illustrated in figure 5, were: rotor

concepts -- one tilt rotor and one stowed rotor design;

propeller concepts -- three design variations of the tilt

wing; lift-fan concepts --three design variations; and lift

jet concepts -- one design. Following are two examples of

VTOL 60-passenger lift-fan aircraft designs

Figure 6 was the Boeing VTOL lift-fan aircraft design.

VTOL DGW was 79,000 ib, or 85,500 Ib if non-burning reaction

control nozzles were used. Cruise speed was Mach 0.80. The

propulsion system had 8 engines, 4 cruise and 4 to power the

gas-driven lift fans. The 4 lift fans were cross ducted in

the roll sense only. Lift fans were 6.45 ft dia, 1.3 PR,

and partial admission scroll arc (163 deg) to facilitate

installation. Low-speed controls had 4 reaction burn

nozzles at aircraft extremities with nozzles also vectoring

for yaw. Sixty percent of control was available without

burn, so the argument was that the complicated (but also

light weight) burning system would rarely be used.

7

Figure 7 was the Lockheed VTOL lift-fan aircraft

design. VTOL DGW was 71,800 ib, and cruise Mach number was

0.80. Each wing tip pod had 3 gas generators, a variable

stator gas-driven lift fan, and a cruise fan driven by a 4-

stage turbine. The 3 gas generators discharged through

isolation valves into a commom manifold to handle engine

out. A concern at the time was whether engines could be

manifolded in this manner. The cruise fan had vectoring

Pegasus-type nozzles. The 1.3 PR lift fan had a diameter of

85 inches. Roll control was a spoilage system achieved by

opposite fore and aft vectoring of the Pegasus nozzles and

the lift fan exit louvers. Roll inertia was high, and roll

control was less then desired. For pitch and yaw the design

featured a turret type double spool valve on the fuselage

aft extremity.

One design eliminated during this civil short-haul

transport study was the pure fan-in-wing. For the fan-in-

wing the same gas generators that drove remote lift fans

during low-speed flight provided thrust for cruise. This

fan-in-wing approach led to large diameter lift fans,

compromised wing design, and a heavy aircraft. The fan-in-

wing was eliminated in favor of the composite lift-fan

aircraft with separate gas generators for cruise flight

(with thrust deflectors for lift in low-speed flight).

A lesson learned was "a red flag of warning" that pure

fan-in-wing designs may not be as competitive as other lift-

fan aircraft configurations.

Figure 8 shows mission areas that were promising for

the VTOL concepts in terms of stage length and payload.

Lift-fan VTOL concepts were promising for 60 passengers at

stage lengths of 500 sm or more. Lift-fan concepts become

more promising as stage length increases. As figure 8

shows,lift-fan concepts also become more promising as

payload is increased. The rate of increase in gross weight

with payload is less for lift-fan VTOLs (and other jets).

For intuitive confirmation of this trend, note the existence

of 747s and comparatively small helicopters and propeller-

driven aircraft. Figure 8 shows no competitive area for

direct lift turbojet VTOLs for civil short-haul, primarily

because of their high perceived noise levels.

One discriminator was gust sensitivity. Lift-fan and

turbojet concepts had the least gust sensitivity due to high

wing loading and low-aspect ratio swept wings.

Another discriminator was perceived noise. High-

frequency noise attenuates with distance more than low-

frequency noise, and lift-fan aircraft generate high-

frequency noise. This led to the result that rotor VTOL

8

aircraft were close to acceptable for city centers but

unacceptably noisy unless many miles from residential

areas. Lift-fan VTOL aircraft were unacceptable for city

centers but acceptable when 2 miles from residential areas.

A lesson learned was caution before concluding lift-fan

aircraft are noisier or quieter than other concepts.

Another lesson learned was that lift-fan aircraft have

time on their side because of the impact of advancing

technology.

In the 1960s, when pushing technology to 1980, improved

propulsion and lighter materials had a more favorable impact

on lift-fan aircraft than on lower disc loading types. If

the 1960s studies were repeated in the 1990s, referring to

figure 8, the 1990s results would show the white area

favoring the low disc loading concepts to be smaller, and

the black area favoring the lift-fan concepts to be larger.

Most of the other relative results regarding the competing

VTOLs would be about the same.

A lesson learned was the sensitivity of short-haul

economics to nonproductive time. In particular, that an

aircraft's deceleration capability during approach, of

importance for many reasons, is also of importance to

minimize nonproductive time.

Lift-fan aircraft can decelerate; for some concepts

deceleration is a limitation. One VTOL lift-fan aircraftB

had a total deceleration, tan _ + u, of 0.58g. The value

0.58g was not used because the component of the deceleration

along the flight path exceeded passenger acceptance. For

non-passenger carrying civil aircraft and for military

aircraft such a large deceleration is a major merit. A

total deceleration of 0.30g was used for the lift-fan

aircraft compared to 0.20g for the tilt wing, and to less

for some STOL types. Such differences yielded less

nonproductive time during landing approach for the lift-fan

aircraft with favorable impact on direct operating cost, as

well as the other advantages such as steeper glide slopes

for terrain clearance and noise reduction.

The STOL concepts, illustrated in figure 9, were:

propeller concepts --two variations of the deflected

slipstream concept; lift-fan concepts -- two variations of

the fan-in-wing concept plus one propulsive wing; and

turbojet concepts -- one jet flap and one EBF (externally

blown flap). As for VTOL, the STOL pure fan-in-wing was

eliminated during the study.

Figures i0, ii, and 12 illustrate a 60-passenger STOL

lift-fan aircraft of a different type, namely the Vought

propulsive wing design. For the i000 ft STOL, DGW was

67,500 ib and cruise Mach number was 0.90. Small-scale windtunnel tests supported the contention that drag rise Machnumber for the propulsive wing design was 0.90. As shown infigure II, four gas generators drove 8 wing-mounted turbineswhich were shaft connected to 8 wing fans of 36.1 inch dia.Two additional gas generators drove two fuselage mountedturbines which were connected by long shafts to two fuselagenose fans. The fuselage nose fans operated in cruise aswell as STOL. Each wing gas generator was interconnected tothe corresponding gas generator on the opposite side by agas duct. During slow speed flight, pitch control wasaugmented by differential thrust between nose and wing fans;and roll and yaw were by differential wing thrust vector and

by differential wing thrust using gas power transfer.

The conclusion for STOL was for a commercial field

length of 2000 feet, propeller, lift-fan, and turbojet

concepts were competitive. For a field length of I000 feet,

the promising STOL concepts were the propeller, lift fan,

and turbojet types in that order.

In the 1960s predictions for advancing technology were

the same for STOL as for VTOL, namely that advancing

technology favors turbomachinery STOLs. During the next

thirty years came the YC-14, YC-15, QSRA, Russian and

Japanese USBs, and the C-17. Today, propeller STOL short-

haul transports can not challenge turbomachinery STOLs, such

as QSRA USBs, even at the shorter field lengths within the

STOL powered-lift category.

As of today, the STOL lift-fan short-haul transport

aircraft has lost out to the STOL USB and EBF concepts. One

point for consideration is as follows. There is no apparent

straight-forward way to evolve STOL USB or EBF designs into

VTOLs, V/STOLs, or STOVLs. STOL lift-fan aircraft, on the

other hand, can be evolved into the other powered-lift

aircraft categories that require vertical flight.

The lesson learned was that the lift-fan aircraft

concept requires I periodicall D review of the lift-fan "family-

of-aircraft" approach. If a "second best" STOL concept uses

some of the same propulsion components as its vertical

flight counterpart, that STOL concept may not be second best

for a total system.

I0

Near-Term V/STOL Lift-Fan Research Transport

In the early 1970s NASA, McDonnell, Boeing, and

Rockwell published design studies on the definition of

candidate V/STOL lift-fan research transports. One of

NASA's contributions was creation of design goals and

criteria specifically for lift-fan research aircraft, which

was different from that for design of operational aircraft.

This led to a lesson learned, as presented below.

McDonnell proposed V/STOL lift fan plus lift/cruise fan

Model 253, a modification of a DC-9, shown in figure 13. It

was powered by six GE YJ97/LF460 engines interconnected in

pairs. The gas generators in the wing tip pods were

interconnected as were opposite forward and aft gas

generators in the fuselage. This candidate research

aircraft design featured an integrated propulsion/low-speed

control system known as the Energy Transfer Control system

which is presented in detail in a later section.

Boeing proposed V/STOL lift fan plus lift/cruise fan

Model R984-33, a modified C-8 Buffalo, shown in figure 14.

It was powered by four GE YJ97/LF460 lift fans. Two gas

generators, located in the fuselage, powered the two remote

tip-turbine lift fans in the wing pods; and two gas

generators in the wing pods powered the two fans located in

the rear of the wing pods.

Rockwell proposed modification of their OV-10 aircraft,

using a lift fan wing pod on each semispan. This candidate

V/STOL research aircraft is not illustrated.

One lesson learned from this lift-fan research aircraft

design study concerned the NASA-developed research aircraft

design goals and criteria. In the interest of minimizing

absolute cost while maximizing research productivity per

dollar, many of the design goals and criteria for research

aircraft are, and should be, less demanding than those for

design of their operational aircraft counterparts.

The lesson learned was to also give consideration to

the opposite case. That is, to ask which of the design

goals and criteria for powered-lift research aircraft should

be tougher than for the operational aircraft.

Example possibilities out of many are (I) higher

control power and/or control response for interpolation

rather than extrapolation of research results, (2) though on

a limited scale, certain critical provisions during design

and fabrication so that the option exists for future

modification of the research aircraft into a variable

stability aircraft of this class, and (3) "excessive" lift

fan thrust vectoring to enable definition of the amount for

the operational aircraft.

II

Conceptual Design of a V/STOL Lift Fan Commercial Short

Haul Transport

This study included NASA in-house activities; contracts

to Boeing, McDonnell, Rockwell, and General Electric; and

assistance from Hamilton Standard. It was conducted during

the early 1970s.

Some of NASA's design goals and criteria were I00

passengers, 400 nm range for VTOL, 800 nm range for STOL

with 1500 ft or less desired, cruise airspeed of 0.75 Mach

no., critical gas generator out, and study of both

philosophies safe-life and fail-safe fans but with emphasis

on designing for a fan failure.

Studied were remote gas-driven, remote mechanically

driven, and integral lift fans. Configurational variants

were lift fan-in-wing pod, and hybrid lift fan-in-wing pod +

lift fan-in-fuselage. All final configurations included

lift/cruise fans on the aft fuselage. After the initial

contracts and additional work, one final configuration had

the lift/cruise fans in wing pods.

Figure 15 shows Boeing Model 984-134 100-passenger

V/STOL integral fan transport design. DGW was VTO 110,200

ib, and i000 ft STOL 119,100 lb. Cruise Mach number was

0.75.

The Boeing design had 8 integral fans, 12.7 bypass

ratio, and 1.31PR. The two integral lift/cruise fans on

the aft fuselage were fixed in cruise position, with Pegasus

nozzles for low-speed flight. Upon engine or fan failure,

the opposite engine was shutoff for balance. Boeing

advocated that civil transports must be designed to handle

both engine and fan failure, a position well received byNASA.

In the Boeing design, figure 15, low-speed attitude

control was provided by varying thrust magnitude and

direction on all 8 integral engines. The integral fans were

operated independently, and for control required both a

rapid response system similar to spoilage systems used on

remote fans, and an rpm change for the longer term effect.

From this Boeing design a lesson learned was not to

make assumptions about operational characteristics of V/STOL

lift-fan aircraft. For the civil transport in figure 15,

with payloads less than Ii,000 ib, VTOL yielded greater

range than i000 ft STOL. This occurred because VTOL and

STOL civil fuel reserve requirements differ, favoring VTOL,

because STOL DGW/VTOL DGW was only 1.08, and because of

internal fuel capacity.

12

Figure 16 shows McDonnell Model VTI02-6-6A 100-passen-

ger V/STOL design. VTO DGW was 109,000 Ib, i000 ft STO DGW

121,300 ib, and cruise Mach number 0.75. Figure 17 shows

the propulsion system layout and gas duct interconnectschematic. The design had six gas generators driving six

fans of 1.25 PR and 87.9 inch dia. Thrust vectoring was by

exit louvers of the four lift fans, and by the aft fuselage

lift/cruise fans by extending and retracting the hood and

also rotating the hood about the fixed lift/cruise fan's

longitudinal axis. A paired interconnect system was used

for gas generator out, and also for fan failure by

distributing gas power to one operable fan and one emergency

backup nozzle located adjacent to the failed fan. The low-

speed pitch and roll control system was based on ETC.

From a contract extension, figures 18 and 19 illustrate

McDonnell Model VTI07-4-4I 100-passenger V/STOL transport

design. This design had 4 engines, and was favored over the

6 engine design, partly because the 4-engine design had

higher dispatch reliability. McDonnell contended that since

all four engines operated for only i0 percent (or less) of

mission time, and only two engines were operated for 90

percent of the time, the dispatch reliability of this V/STOL

would be higher then that of a 4-engine CTOL.

For the 4-engine design (figure 18), VTO/STO DGW was

113,000/125,400 ib, and cruise Mach no. 0.75. The aircraft

was designed for gas generator or fan out. The fans had

1.39 PR and a 97.9 inch dia. The normally "inactive"

interconnect ducts were of 18.3 inch dia and during engine

out the duct hot gas flow maximum Mach number was 0.4.

The 4-engine design used two positions of thrust vec-

toring during STO; best angle for ground roll acceleration

(23 deg), and best angle for liftoff and climb to 35 ft (53

deg). (Even for powered-lift aircraft that can vector all

thrust horizontally, maximum ground roll acceleration does

not occur when the thrust is pointed straight down the run-

way, because small angles lighten gear loads and reduce hor-

izontal thrust imperceptibly.) At the end of the STO

mission the aircraft had the capability to land vertically.

The ETC system modulated thrust 28 percent for pitch

and 25 percent for roll control. Fuselage fan exit louvers

provided yaw, with the greatest deflection (21 deg) needed

at minimum vertical landing weight. Typically V/STOL

aircraft are more difficult to control at the lighter gross

weights because of less fan thrust magnitude available.

Figure 20 shows Rockwell 100-passenger design called in

their reports the Remote Fan/Turbojet V/STOL Transport. VTO

DGW was 120,000 lb. STO DGW was 132,000 ib, but STO at that

weight required a field length of a little more than 2000

ft. Cruise Mach number was 0.75.

13

The Rockwell design had 8 gas generators driving 8

fans, 6 lift fans and 2 lift/cruise fans with vectoring

hoods. The propulsion system featured paired interconnected

systems, thus there were 4 paired systems. A typical paired

system is shown in figure 21. Figure 22 is a propulsion

system schematic that shows overall layout and fan failure.Note that a fan failure led to shutdown of a second fan, and

then the gas generator exhaust flow was directed to 2

emergency exit nozzles, one each located near the failed and

shutdown fans. Cruise thrust was provided in part by an

unusual feature. To augment cruise thrust from the 2

lift/cruise fans, 2 additional gas generators (labeled A and

D in figure 22) were "converted" into cruise turbojets by

directing their exhaust flow through convergent nozzles.

In the Rockwell design pitch and roll control were by

ETC thrust modulation. Yaw was by differential fore and aft

deflection of the thrust from all six wing pod lift fans.

Viewing the three contractual design studies

collectively, the 100-passenger V/STOL designs with integral

fans and those with remote fans were competitive.

One lesson learned from this lift-fan aircraft design

activity was that for V/STOL aircraft (less so for STOL,

VTOL, or STOVL aircraft) the design implications differ for

military and civil applications.

For military aircraft, the STO gross weight is greater

than the VTO gross weight with structural, gust sensitivity,

and other design criteria typically based on the VTO gross

weight. Thus, for STO missions the military V/STOL is an"overloaded" VTO aircraft.

For civil aircraft, overloading is not permitted.

There are several design options available (see Appendix I

for additional discussion). For these design studies the

contractors selected the design option that follows.

The civil V/STOL aircraft performed a VTOL mission

although the useful load for VTO was compromised by the STO-

determined structural weight. For this civil design option,

the STO gross weight can be "too much" as well as "too

little". The STO/VTO gross weight ratio is the design issue

for this class of civil V/STOL aircraft.

For example, McDonnell chose to increase VTO gross

weight from 109,000 ib as required for the VTOL mission to a

compromised VTO gross weight of 110,800 lb. Thus for the

STO gross weightjstructure was sufficient to maintain othervalues, such as high airspeeds for the STO mission legs.

Other compromises such as placarding airspeeds when at the

STO gross weight were not necessary.

14

Another lesson learned was that for lift-fan aircraftthat can operate in more than one mode, in this case V/STOLthat can operate VTOL or STOL, optimum in-flight require-ments may differ between modes, and thus the aircraft mustbe designed to be non-optimum with respect to at least oneof the modes. An example from these studies follows.

For this study, design ranges were 400 nm VTOL and 800nm STOL. One tradeoff study result was that though a cruiseMach number of 0.75 was optimum for the VTOL mission, a Machnumber of 0.80 was needed for the STOL mission. It wasargued that, to generate sufficient production base,potential operators would have to be provided with 0.80cruise Mach number for the STOL mission in order to competewith and be compatible with CTOLs. So a heavier and morecostly aircraft resulted for the VTOL mode to accommodatethe in-flight STOL requirement.

The lessons learned have programmatic implications. Toillustrate, suppose it is proposed to develop a STOVLaircraft. The "best" STOVL will probably be non-optimum sothat the total program can be optimum. The total programwill consist of any number of elements selected from alaundry list such as follows.

* A STOVL aircraft operated only STOVL.

* A STOVL aircraft operated STOVL and STOL.

In addition to a STOVL, a STOL variant that is amodest modification of the STOVL and operated onlySTOL.

* A STOL variant operated STOL and CTOL.

* A CTOL variant operated CTOL.

The STOVL designers will design the best slightly non-optimum STOVL if they understand what aircraft you areinterested in and to what degree.

15

Conceptual Design Studies of a V/STOL Military-Civil LiftFan Aircraft

In the early 1970s a McDonnell in-house activity

yielded a 3-fan design known as the Model 260. Past NASA

civil-oriented design activities and the McDonnell Model 260

design study provided the base for the first NASA-sponsored

lift-fan aircraft contractual design study that included

military applications.

In 1973 NASA awarded contracts to McDonnell, Rockwell,

and GE for the conceptual design of a military lift-fan

aircraft. Unlike post-1973 design efforts, this first

effort did not give consideration to military multimissions,

but rather focussed on the one mission, Vertical-On-Board

(VOD) delivery. The VOD mission was for intrafleet and

shore-to-ship logistics support.

The design mission was STOVL, to transport a VOD

payload of 5000 Ib a distance of 2000 nm at a cruise Mach

number of at least 0.75. STO was engine-out, 400 ft, and 20

kts wind-over-deck (WOD). Though some VTOL capability was

desired, VTOL was not specified but rather accepted as a

design fallout.

The McDonnell Model VT I06-3-3D design, shown in

figures 23 and 24, had 3 remote gas-driven lift fans of 1.40

design pressure ratio. Number of gas generators was 3 for

"fail safe" and 2 for "safe life" design philosophies.

Design gross weights for "fail safe" (engine-out) were

40,000 ib STOVL and 28,000 Ib VTOL.

One lesson learned was the fundamental compatibility

between military lift fans and civil lift fans, despite the

fact that civil lift fans are compromised by noise level

constraints. The study showed there was little difference

in overall aircraft design and performance if using pure

military lift fans or if using civil lift fans with those

noise reduction features of the fan that were easy to remove

stripped out. A military lift fan, with straight forward

modifications, can be utilized by the civil community, orvice versa.

The Rockwell VOD design had 4 two-stage gas-driven

fans of 1.5 pressure ratio and 4 engines as shown in figure

25. Design gross weights for engine-out were about 40,500

ib STOVL and 30,000 ib VTOL. Some design features were (I)

single swivelling nozzle on each nacelle mounted lift/cruise

fan as shown on figure 26, (2) use of two-stage fans, see

figure 27 for a drawing of the two-stage fan and comparison

to a single-stage fan, and (3) quad entry lift fan scrolls

as shown on figure 28 including comparison to a dual entryscroll.

16

Two-stage fans become of interest for high speed

subsonic STOVL aircraft when the cruise leg is long, in this

case 2000 nm.

In the quad entry scroll the added entries supplied one-

sixth of the flow each, thus allowing the two primary

entries and associated scroll cross section diameters to be

smaller. The added entries can be designed with various

orientations with respect to the scroll to suit the needs

for individual installation requirements, as illustrated by

the alternate location in figure 28. Quad design reduced

the overall planform dimensions of the fans from 4 to 7

inches. Dual and quad fan weights differed by i0 Ib in

favor of dual. The main advantage of the quad was bene-

ficial effect on nacelle installation weight and drag.

Should a gas-driven lift fan mounted in the fuselage of

a STOVL supersonic fighter, wherein fuselage fineness ratio

is at a premium, have a quad entry scroll?

17

Design Definition Study of a Lift/Cruise Fan Technology

V/STOL Aircraft, Part I, Navy Operational Aircraft

This was a national activity with NASA in-house effort;

NASA/Navy contractual studies by Boeing/Allison/ Hamilton-

Standard, McDonnell/GE, and Rockwell/GE; Navy contractual

studies; and contractor in-house studies by nearly all U.S.

aircraft airframe and engine companies. The first of many

NASA CRs was published in 1975.

This was the first design in which the task was a lift/

cruise fan V/STOL aircraft for Navy multimissions. Five

STOVL missions were required, namely Anti-Submarine Warfare

(ASW), Surface Attack (SA), Combat Search and Rescue (CSAR),

Surveillance (SURV), and Vertical On-Board Delivery (VOD).

Highest priority was the ASW mission, cruise out 150 nm, 4-

hr loiter at i0,000 ft, and return.

Though STOVL was the primary mission, the aircraft had

some V/STOL capability. STO was for 400 ft with I0 kts WOD.

Critical engine out (but not fan out) was a requirement.

There were no design guidelines for civil missions.

With minor modification the designs would yield civil

aircraft to support off-shore oil rigs and other civil

utility missions.

Design configurations included gas-driven and mechani-

cally-driven fans, 2 and 3 engines, 2 lift/cruise fans, with

and without a nose fan. A typical configuration had a STOVL

DGW of about 38,000 ib and a cruise Mach number of 0.80.

The Boeing Model 1041-133 had mechanically driven and

interconnected lift fans, 2 integral rotatable lift/cruise

fans on the aft fuselage plus a fuselage nose lift fan. See

figures 29 and 30. Design illustrations follow.

Figure 31 is a power train schematic. The clutch was

used to isolate a failed engine or to permit loiter on one

engine.

Figure 32is the lift/cruise fan engine pod. Note blow-

in inlet doors for low-speed flight, Hamilton Standard 62

inch diameter variable pitch fan thats use included pitch

and roll control, the fan variable exit area nozzle that

reduced area to 70 % for cruise, the yaw control vanes, and

the Allison T701 engine.

Figure 33 includes the structure and mechanisms used to

pivot the two aft lift/cruise fan nacelles.

Figure 34 shows the drive shaft that connected the

forward fuselage nose lift fan, through a clutch, to the

main fan drive gear box.

18

The McDonnell gas-driven lift fan plus lift/cruise fan

design is shown on figures 35 and 36. More information is

not presented in this section because another McDonnell

design that is fundamentally similar is discussed later.

The Rockwell gas-driven design without a nose fan and

with puffer pipes for pitch control is shown on figure 37.

Unlike Rockwell's design for the single mission VOD aircraft

that used two-stage fans, for this multimission design

Rockwell chose two 1.3 pressure ratio, single stage, 60 inch

dia, lift/cruise fans. The design had 3 J97-GE-100 gas

generators. Roll control was by ETC, yaw by differential

operation of the nacelle single swivel nozzles, and pitch by

puffer pipes powered by a third gas generator which also was

available for auxiliary horizontal turbojet thrust.

One lesson learned is the flexibility available to the

aircraft designer if lift fans are interconnected. The

interconnect can be by gas-driven interconnect duct or by a

mechanical shaft. Design flexibility is of more importance

for multimissions than for single mission designs. With

interconnect, straight-forward aircraft design variants are

an option. For example, the same basic multimission design

in this study had 2 or 3 gas generators as a function of the

specific mission.

Another lesson learned concerned gyroscopic coupling.

Consider the configuration that featured two aft fuselage

mounted lift/cruise fans that were rotated for conversion

(figure 30). Gyroscopic coupling in the vertical and low-

speed flight modes occurred whenever the aircraft pitch orroll attitude was varied, or whenever the nacelle incidence

was varied. When the entire aircraft pitched, all three

fans contributed to gyroscopics. If only nacelle incidence

was varied for conversion, then only the two aft lift/cruise

fans contributed. The fans and engines were rotated in

opposite directions to reduce total angular momentum.

For this study a guideline was that gyroscopic moment

of less than I0 percent of the available control was

considered acceptable. Example results were, despite use of

opposite rotation, that nacelle incidence rate was limited

to 22 deg/sec, and aircraft roll attitude rate was limited

to II deg/sec. Not only must the designer minimize

gyroscopics, the customer and the designer must have an

understanding of what the gyroscopic design criteria are in

the first place.

For a STOVL design with one 2-stage lift fan in the

fuselage, should the 2 stages counter rotate?

19

Design Definition Study of a Lift/Cruise Fan Technology

V/STOL Aircraft, Part II, Technology Aircraft

Paralleling the preceeding section on Navy multimission

aircraft design were studies to define research aircraft,

also known as technology aircraft, and also know as research

and technology aircraft and therefore by the acronym RTA.

The NASA/Navy RTA studies included three approaches to

design of the research aircraft, namely (i) new airframe --

full flight envelope, (2) modified existing aircraft -- full

flight envelope, and (3) modified existing aircraft --flight

envelope limited to low-speed capability.

Most effort was placed on modification of existing

aircraft. All three airframe contractors selected modifi-

cation of the Rockwell Sabreliner T-39 business jet. Two of

the selected designs featured gas-driven fan systems, and

one featured mechanically driven fans. All had three

existing engines and two lift/cruise fans; and some had a

fuselage nose lift fan. VTO design gross weights were in

the 25,000 to 30,000 ib category.

An isometric of the McDonnell Model 260-RTA-I is shown

in figure 38, and some propulsion/control system details are

in figure 39.

Over the years there have been many different lift fan

scroll designs proposed. The scroll on the McDonnell RTA is

shown in figure 40. It is known as the Scroll-in-Scroll

concept. It consists of an outer scroll of 2/3 arc and an

inner scroll of i/3 arc. During normal operation both the

ETC and 1/3 scroll valves are open, to provide 100% arc ad-

mission. For engine failure, the I/3 scroll shutoff valve

is closed and thus only a 2/3 arc is utilized. This Scroll-

in-Scroll design was used on both the nose lift fan and on

both aft lift/cruise fans. When initiating gas-driven lift

fan design, review the types of scrolls already studied.

The McDonnell RTA low-speed control system used, as an

integrated part of its ETC system, thrust spoilage systems

on both the nose lift fan and the two aft lift/cruise fans

as shown in figures 41 and 42. The name of these systems

became Thrust Reduction Modulation and the acronym TRM.

A lesson learned concerned an attribute of a control

system with TRM. Previously understood was that TRM

quickened control response and prevented control coupling.

This RTA design exercise included military considerations,

which led to the following lesson learned.

2O

The complementary functions of ETC and TRM provided

an inherent safety feature by nature of the separate

actuation of these devices at each fan. Loss of an ETC

function did not interfere with TRM operation, and

conversely loss of the TRM did not interfere with ETC

operation. This feature thus provided survivability when

multiple failures or battle damage were considered. When a

total loss of a TRM or ETC function at a fan occurred,

adequate aircraft control was maintained with some

degradation in handling qualities.

A 3-view drawing of the Boeing Model I041-135-2A RTA is

shown in figure 43. Except for one difference as discussed

below, this modified T-39 was very much like the Boeing Navy

multimission aircraft presented in the preceeding section.

One major design difference, not shown in figure 43, was

that a third engine was added, making it a mechanically

driven 3-fan, 3-engine aircraft. To improve engine out

thrust-to-weight margins for the T-39 RTA, a third Allison

XT-701 engine was installed inside the fuselage aft of the

center wing section. Modifications for the third engine

included addition of a drop box (helical gear), drive shaft

connecting the third engine to the drive system drop box,

and fuselage inlet and exhaust ducting. Unlike the engines

driving the lift/cruise fans, the third engine, being

interbody mounted, was unsupercharged.

The final 3-fan, 3-engine gas-driven McDonnell T-39 RTA

and the final 3-fan, 3-engine mechanical Boeing T-39 RTA

were considered competitive.

Another T-39 RTA, designed by Rockwell, is shown in

figures 44 and 45. It had the unusual feature of a third

engine that was not normally used to drive either a lift fan

or a lift/cruise fan. The third engine, during low-speed

flight, powered the pitch axis puffer pipe system (see

figure 45) and was on standby to power lift/cruise fans in

the event of a lift/cruise fan engine failure.

In the interest of military and civil future aviation,

the author was (and still is) disappointed that none of the

RTA designs ever reached flight status.

21

Contractual Lift-fan Aircraft Design Studies During the

Period 1978-1992.

There were no NASA contractual lift-fan aircraft design

studies during this 15 year period. NASA continued to

conduct basic and applied research on lift-fan aircraft

technology. For completeness presented are two designs

whose origins were due to contractor and Navy efforts, and

for which NASA conducted research activities.

Figure 46 shows a large scale powered model of the

Grumman V/STOL twin tilt nacelle design with mechanically

interconnected integral lift fans (i.e. high-bypass

turbofans). Figure 47 is a schematic of the propulsion

system including the vanes for low-speed control.

Figure 48 shows the McDonnell V/STOL twin nacelle

design with mechanically interconnected fixed turbofans.

Thrust vectoring was by "vented D" nozzles. Figure 48 is a

schematic of the propulsion/control system.

NASA research pertaining to these 2-engine V/STOL

turbofan designs is highlighted in other papers of this

series.

22

Design Integration

An important subject in lift-fan aircraft design is

design integration. Which design has the best propulsion,

low-speed controls, or structural weight fraction is less

important than which design has systems and structure

integrated to yield the best overall aircraft.

In addition to NASA contractual lift-fan aircraft

design studies were many NASA contractual experimental

investigations to validate aircraft designs. Three full-

scale experimental investigations were selected for

examples, namely (I) the behavior of gas generators when

plumbed together by common manifold interconnect ducting,

(2) the design, fabrication, and test of hot gas

interconnect ducts, and (3) the integrated propulsion/low-

speed control system known as Energy Transfer Control (ETC).

Manifolding of Gas Generators

Studies of lift-fan V/STOL transports and of research

aircraft included designs with a pair or more of gas

generators interconnected to drive a pair or more of remote

fans. An example of a lift-fan V/STOL transport design that

featured three interconnected pairs is shown in figure 13.

One research activity was a full-scale experimental

investigation in which two GE YJ-97 gas generators were

interconnected by a 50-foot gas-transfer duct with an ID of

14 inches, shown schematically in figure 50. YJ-97s were

single-stream turbojet gas generators each with a rated

thrust of 5200 lb. This contractual effort performed by

McDonnell included a total of about 120 hours of gas

generator operation.

The interconnect duct was designed for one-half of the

rated exhaust gas flow from one gas generator. The duct gas

flow Mach number was 0 with both gas generators operating

and 0.4 Mach number with one gas generator failed.

The investigation demonstrated that the system was

tolerant to differential gas generator speed conditions.

Though normally o_ _r_t_d at iOcntical throttle settings,

since differentials occur, tests were conducted to evaluate

system stability during differential transient throttle

operations. One gas generator was throttled to a lower

speed. The low-speed gas generator recovered satisfactorily

from all conditions, including from 80% speed with the other

gas generator at rated speed, and from 65% speed with the

other gas generator at 97% speed.

23

To investigate emergency operation (see figure 51), the

simulated gas generator failure sequence was as follows.

The throttle on one gas generator was abruptly chopped.

Closure of the isolation valve was initiated, followed byinitiating closure of one of the two lift-fan shut-off

valves. The valve closure rates and the delay betweenthrottle cutoff and initial valve movement were varied to

determine system sensitivities.

Figure 52 is a time history of a throttle cutoff on gas

generator no. 2. Gas generator no. 1 speed remained steadyduring and after cutoff of gas generator no. 2. Transition

to the stabilized one gas generator failed state was

completed in 6 seconds. The 2.5-second delay in initiating

isolation valve closure did not result in reverse flow

through the no. 2 gas generator.

Lessons learned were that the paired interconnected

system was insensitive to transients, even to delays in

recovering from gas generator failure. Transient time

requirements for recovering from gas generator failure will

depend on flight requirements. The time that transient lift

loss can be tolerated in flight will dictate design and

valve closure rates rather then concerns of gas generator orother propulsion component sensitivities.

Recommended for manifolding of gas generators isBibliography number II.

Interconnect Ducting

The experimental investigations of paired intercon-

nected gas generators presented an opportunity for evalua-

tions of full-scale interconnect duct segments. Flight-type

metal and semi-flexible composite duct segments were

designed, fabricated, and inserted in the boiler-plate

interconnect duct. Figure 53 shows a conceptual drawing of

the composite duct segment. Figure 54 is the duct wire

wrapped screen liner. Figure 55 shows the final two

components ready for assembly into the full-scale composite

duct segment. The flight-type segments were subjected to

control and engine-out cycles. An example engine-out

condition was gas temperature 1460 degrees F, gas pressure41 psia, flow Mach number 0.4, and time duration 4 minutes.

Figure 56 shows an example time history during a simulated

engine-out condition (the most severe case). Metal and

composite duct segments were promising. The semi-flexible

composite ducts offered advantages of weight, by weight per

length of the duct and by the elimination of heavy ductconnecting elements such as bellows.

For interconnect ducting see Bibliography number 12.

24

Energy Transfer Control

Discussed is an example of integrating the propulsion

system with the aircraft low-speed control system to the

degree that both systems lose their individual identities.

The result is one system with a name such as integrated

propulsion/aircraft low-speed attitude control system.

Lift-fan aircraft low-speed control systems have been

designed with direct gas generator modulation, fan exit

louver thrust spoilage, fan variable area scroll, variable

inlet guide vanes, variable blade pitch, control vanes in

the exhaust fan flow from lift/cruise fans, variable (fan

exit) area control system known as VACS, gas generator

exhaust modulation known as turbine energy modulation (TEM),

and an exhaust modulation system chosen for this paper known

as the Energy Transfer Control (ETC) system.

ETC was pioneered by McDonnell, and investigated by

NASA in-house and sponsored activities. Figure 57 is a

general arrangement of a paired ETC system. The basic

attribute of ETC is that control moments are generated by

capitalizing on short duration (fractions of a second up to

a few seconds) transients of the propulsion system. This

approach avoids penalties with installed gas generator power

and the lift-fan steady-state thrust design point.

An experimental investigation was initiated using the

pair of interconnected YJ-97 gas generators shown in figure

50. Figure 58 shows the full-scale, 14 inch dia ETC valves,

and figure 59 shows the full-scale interconnect duct shutoff

valve. Lacking full-scale tip-turbine lift-fan hardware,

the lift-fans were simulated by using full-scale lift-fan

designs. That is, the real gas generator exit nozzle gas

energy data, steady-state and transient, were used to

predict the behavior of the lift fans.

A result is illustrated in figure 60, with both gas

generators operating near rated power and no. 1ETC valves

deflected 40 degrees. Shown are the gas-flow character-

istics at the simulated entry to the lift-fan scrolls. The

gas energy at the no. I exit nozzles remained nearly con-

stant while the gas energy at the no. 2 exit nozzles in-

creased by a factor of 1.64. The no. I exit nozzle gas

energy remained constant because the increase in gas temp-

erature and pressure upstream of the ETC valves offset the

pressure drop across the deflected ETC valves. The no. 2

exit nozzle gas energy increased greatly because tempera-

ture, pressure, and gas flow all increased as in figure 60.

25

An example of ETC with lift-fan thrust calculated from

measured gas energy data is shown in figure 61. Total fan

thrust increased as the ETC valves were closed. System

design included fan exit louver vanes. The louvers for a

fan were activated only when the ETC valves for that same

fan were activated. The louvers were used to spoil the

thrust of the "constant" thrust fan. The purposes of the

louvers were to spoil thrust to (I) generate large control

moments, (2) decouple attitude control from height control,

and (3) improve the aircraft first-order control moment time

constants. Fan thrust modulation of 25% at takeoff power

and 80% at landing power was achieved, and control moment

time constants were 0.14 second at takeoff power and 0.25

second at landing power. These values met design goals.

The initial configuration for gas generator failure is

shown in figure 62. Gas generator no. 2 has failed, and the

isolation valve and ETC shutoff vaives have been closed.

Gas flow was distributed so both fans remained in operation,

no asymmetric moments were generated to upset aircraft

attitude, and transient fan thrust was provided by

modulating one ETC valve at each fan location as required.

Figure 63 shows a transient thrust time history for a

30 degree step input of ETC valve no. 1 with no. I gas

generator at 97.5% speed. Incremental thrust is presented

for fan no. I, for fan no. 2, for differential lift change

(fan no. 2 -fan no. I) as required for attitude control, and

for total lift change (fan no. 2 + fan no. i) to illustrate

height control coupling with attitude when a gas generator

has failed. For the emergency condition of a gas generator

failure, these characteristics were satisfactory.

At the time ETC was investigated, it was in the context

of being applicable to a short-haul transport aircraft. One

of the configurational variants had six gas generators, sixremote lift fans, and 21 valves. ETC became associated with

lift-fan aircraft configurations that have many gas

generators, many lift fans, and many valves. A lesson

learned was that ETC is also applicable to lift-fan aircraft

designs that have few gas generators, few lift fans, and few

valves. Three examples follow.

Example no.l: Instead of one of three paired systems

in a large transport, the ETC just described can be all of

the system, applicable to a configuration with two gas

generators, two lift fans, and seven valves (figure 57).

26

Example no. 2: Figure 60 is for all gas generators

operating, and the gas generators interconnected and

operating as a team. The experimental data base remains

valid regardless of the number of gas generators assumed to

be on line. The simplest case for all gas generators

operating as a team is to have one, and since only one, no

interconnect. Thus the data base is applicable to a

configuration with one gas generator, two lift fans, and two

valves. Two valves because gas generator isolation valves

are not needed, nor an interconnect valve, and since the

pair of ETC valves at each fan always operate in parallel,

the number of ETC valves can be reduced to one per lift fan.

Example no. 3: For the civil transport, consideration

was given to lift-fan failure. The design included an

emergency exit nozzle located near each lift fan. Upon lift-

fan failure, the propulsion/ETC system distributed flow to

the emergency exit nozzle (more than one-half of the flow)

and to the other on-line operating lift fan. This design

approach worked well. This suggests an alternative design

in which the exit nozzle is not an "emergency" nozzle. ETC

can apply to a configuration having one gas generator, one

lift fan, and one exit nozzle. Perhaps all three components

are located on the aircraft's longitudinal axis, with the

integrated system providing height and pitch and yaw

control, with roll provided separately. This ETC design

needs two valves, one upstream of the one lift-fan scroll

and one upstream of the exit nozzle.

The point to the preceding examples of ETC systems with

fewer components is not whether these examples have merit.

The point is the potential wide applicability of integrated

propulsion/ETC systems.

A further point is that the advantages of ETC improve

as lift-fan pressure ratio increases. Past studies were

based on fan pressure ratios from 1.25 to 1.40. As

technology enables higher fan pressure ratio, or if the

current lift-fan aircraft under study is conducive to use of

higher lift-fan pressure ratio, ETC becomes increasingly

attractive.

NASA Ames has an extensive ETC data base. The lesson

learned is to ask whether ETC is applicable to the lift-fan

aircraft configurations currently being addressed, and to

stay knowledgeable with respect to the ETC data base.

For full-scale experimental investigations of ETC, see

Bibliography number II.

27

The Avrocar Flight Evaluations

On two occasions, in 1960 and 1961, the USA VZ-9AVAvrocar was evaluated in a series of flights by a two-manUSAF team, the project pilot and the author.

The Avrocar was manufactured by Avro Aircraft, Limited,Malton, Ontario, Canada. Figure 64 is a photograph of theAvrocar in hovering flight. Figure 65 is an artisticschematic of the aircraft.

The circular planform Avrocar had a wing span(diameter) of 18 feet. The wing section was symmetricalabout the vertical centerline, elliptical in profile, with a

thickness/chord ratio of 20%. Gross weight was 5650 pounds.

The aircraft was powered by 3 Continental J69-T-9 gas

generators rated sea level static thrust of 927 pounds each.

One lift fan was located in the center of the fuse-

lage. The Orenda lift fan was a single stage axial flow

fan. Fan inner and outer diameters were 20 and 60 inches.

The 31 fan blades had a blade chord of 4.1 inches, a hub and

tip thickness/chord ratio of 13% and 8%, and a tip Mach

number of 0.78. Weight of the fan, with stator, shroud, and

seal, was 338 pounds. The 124 turbine blades had a chord of

2 inches, Jand the turbine blade tip diameter was 65 inches.

Though production aircraft were envisioned of higher

performance, the VZ-9AV research aircraft was originally

proposed as capable of VTOL, and flight to i0,000 feet at an

airspeed of 200 mph. Up-and-away flight was never achieved

because of an unstable ground effect height, insufficient

thrust-to-weight ratio, and inadequate low-speed control.

Of the many lessons learned from the Avrocar FlightEvaluations, ten are selected herein as follows.

I. The decision was made to fly first, and later to

put the aircraft in NASA's full-scale 40 x 80 foot wind

tunnel. That sequence of events was backwards.

2. The gas generator thrust, 3 x 927 = 2781 pounds,

augmented by the lift fan, should have enabled VTO at 5650

pounds gross weight, but it didn't. Duct design was

deficient; holes in the ducts for control cables were not

sealed, duct contour was compromised, and "transition" doors

in the ducts were not sealed. Lift-fan thrust augmentation

was offset by duct losses. Duct design is as important as

gas generator or lift-fan performance.

3. The low-speed aircraft attitude control system

originally was a spoiler system that was later replaced with

a focussing ring control system as seen in figures 66 and

28

67. The design approach to transition from the low-speed

control system to the high-speed control system is

illustrated in figure 68. (The transition was never

attempted in flight.) The original spoiler control

produced pitching and rolling moments by destroying lift onone side and not creating more lift on the opposite side.

Thus attitude control and height control were severely

coupled, and total lift loss during attitude control was

very high. The decision, in 1959, to abandon the low-speed

control system based on spoilage was a lesson learned. In

design of lift-fan aircraft capable of vertical flight,

effort should be directed at developing low-speed control

systems that do not feature thrust spoilage. The second

system installed on the Avrocar, the focussing ring system,

featured little loss of thrust.

4. As shown in figure 69, the Avrocar exhibited two

distinct types of air flow distribution in ground effect.

Each type was stable, but transitioning between the two

types was unstable. The Avrocar was unable to continue

vertical takeoff through the unstable area, called the

critical height, and the aircraft became a ground effects

research vehicle. In figure 69, the flow changed from

"curtain" flow to "tree-trunk" flow during a 6-inch vertical

height change. At the unstable critical height, the Avrocar

went into a severe oscillatory mode, that did not go

divergent, that was named "hubcapping". When testing models

or aircraft in ground effect, proceed in small height

increments and/or use dynamic testing techniques. Despite

the experience with the Avrocar and its unacceptable thrust-

to-weight ratio, dynamic vertical flight operations may

overcome apparent barriers such as a steady state hovering

instability at a certain ground height.

5. The Avrocar was symmetrical, longitudinally and

laterally. The lift fan was located in the center of the

aircraft. Despite geometry, the Avrocar was not

symmetrical. A rolling moment and side force of high

magnitude resulted from intake flow entering the lift fan

non-vertically. A ST0VL design with the lift fan on the

fuselage centerline is not symmetrical. In model testing,

simulate the lift fan with another lift fan, and/or use the

"real" lift fan at full scale. Simulating mass flow with an

ejector or other device is, unfortunately, symmetrical.

6. In the Avrocar most of the mixed fan flow exited

through an annular nozzle located about the outer edge of

the circular planform. To help control ground effects, some

of the air exited from an inner row, and some from an outer

row, of bottom surface peripheral jets. In a STOVL lift fan

design, particularly for a design wherein thrust-to-weight

ratio is determined by in-flight requirements and not by VL,

if necessary a ground-effects problem might be overcome by

utilizing bottom surface auxiliary jet(s).

29

7. The Avrocar made one consider both sides of lift-

fan gyroscopics. Adverse gyroscopics are well known;

favorable gyroscopics less know. By using spherical

bearings, the lift-fan hub was free to move by I/4th of 1

degree. This small movement was amplified by mechanical

linkage to provide input to the control system. The result

was a low-speed automatic stability augmentation system that

was inherent to the design and worked well. The lesson

learned is to ask all questions, including whether

gyroscopics should be harnessed in some manner to achieve afavorable result.

8. The Avrocar demonstrated that mixed fan and turbine

-drive air is relatively cool. Hovering over dead grass did

not start a fire and did not scorch the dead grass. During

steady-state hover, environmental problems included

recirculation, reingestion, reduced visibility, mud and

other deposits degrading the wing airfoil contour, etc.

These problems were eliminated by maintaining i0 knots

forward speed. Operationally, VL may not mean vertical

landing; VL may mean almost vertical landing.

9. The lift fan, fabricated in 1958, was one of the

first lift fans installed in an aircraft that was intended

for up-and-away flight. During hover and low-speed flight

over several types of unprepared terrain, the lift fan was

subjected to considerable FOD reingestion. Despite being a

"pioneer" and despite rough environmental treatment, the

lift fan performed well. This was the first lesson, to be

followed by more lessons, that lift fans are tough and

dependable.

I0. The Avrocar cockpit got hot. On a cold day we

shortened a flight because of cockpit heat. In a

configuration with a gas-driven lift-fan in close proximity

to the cockpit, give attention to insulating the

cockpit. The noise level in the Avrocar cockpit was high.

Far-field noise was low, near-field noise was high. The

depth-of-installation of the lift fan was reasonable, (20%

airfoil), but no attempt was made to reduce noise.

Particularly for fan-in-fuselage or fan-in-wing pod designs

with installation depth, spend effort andcommit pounds to

incorporate noise reduction.

3O

Concluding Remarks

Much was and wasn't done.

Iteratively, technology was advanced, aircraft weredesigned, designs were validated with ground-based investi-gations, and technology was advanced.

And for 30 years no one built a lift-fan aircraft.

No lift-fan aircraft was built despite a history of XV-5A/B, excellence in R & T base, creditable advocacy on na-tional need, and creation of interagency partnerships.

On one occasion NASA and Navy agreed on a lift-fan tech-nology aircraft project, subject to approval by Office ofPresident of the United States. The office agreed withjustification advocacy, technical plan, management plan, andfiscal plan, with one exception. Though total funding wasrealistic, NASA and Navy were tasked to reallocate shares,with Navy's increased. Navy could not increase their fund-ing share, nor, by direction, could NASA offer their origi-nal share. Result -- cancellation. Navy cancelled projectfunds were redirected to improve NASA Ames simulators whichwere deficient for vertical flight.

Priorities, in order, for future years are:

I. Build lift-fan research and technology aircraft.Projects exercise the too-inactive contractual design teams;augment related R & T base; include aircraft fabrication,ground-based qualifications, flight technology demonstra-tion, and long-term flight research; and for mature tech-nologies like the lift fan, are the mechanism needed forintroduction of the technology to application.

2. Build full-scale flightworthy or flight-type liftfans and lift/cruise fans, and critical propulsion compo-nents. Arguably, the devastating technical deficiency inpast proposed research aircraft projects was the non-exis-tence of lift fans. Experience is needed to validate lift-fan weights, performance, cost, polar moment of inertia,acoustics, and more. And to sell aircraft projects. Anymilitary or civil activity will benefit the other. For ex-ample, a 2-stage high pressure ratio fan-in-fuselage for aSTOVL supersonic fighter will also enable a civil super-

sonic lift-fan business jet.

3. If numbers 1 and 2 above are, temporarily, not to

be, then determine which national ground-based facility is

deficient for advancing lift-fan aircraft technology, and

fix it.

31

Bibliography

I. Hickey, David H., Preliminary Investigation of the

Characteristics of a Two-Dimensional Wing and Propeller With

the Propeller Plane of Rotation in the Wing-Chord Plane."

NACA RM A57F03, 1957.

2. Holzhauser, C.A.; Deckert, W.H.; Quigley, H.C.; and

Kelly, M.W., "Design and Operating Considerations of

Commercial STOL Transports." Paper 64-285, Ist AIAA Annual

Meeting, Washington D.C., June 1964.

3. Deckert, W.H. and Hickey, D.H., "Summary and Analysis

of Feasibility-Study Designs of V/STOL Transport Aircraft."

Paper 67-938, AIAA 4th Annual Meeting and Technical Display,Anaheim, CA, October 1967.

4. Fry, B.L. and Zabinsky, J.M., "Feasibility of V/STOL

Concepts for Short-Haul Transport Aircraft." The Boeing

Company, NASA CR-743, 1967.

5. Marsh, K.R., "Study on the Feasibility of V/STOL

Concepts for Short-Haul Transport Aircraft." Ling-Temco-

Vought Inc., NASA CR-670, 1967.

6. Anon, "Study on the Feasibility of V/STOL Concepts for

Short-Haul Transport Aircraft." Lockheed Aircraft Corpora-

tion, NASA CR-902, 1967.

7. Anon, "Study of Aircraft in Short Haul Transportation

Systems." NASA CR-986, 1967.

8. Novak, L.R., "A Low Risk Approach to Development of a

Quiet V/STOL Transport Aircraft." McDonnell Aircraft Co.,

Paper No. 70-1409, AIAA 7th Annual Meeting, Oct. 1970.

9. Anon, "Design Study of V/STOL Lift-Fan Research

Transport, Final Report, Part I, Task II." The BoeingCompany, NASA CR-I14549, 1972.

i0. Anon, "Study of Near-Term V/STOL Lift-Fan Research

Transport." North American Rockwell, NASA CR-I14542, 1972.

II. Anon, "A Full Scale Test of a New V/STOL Control

System Energy Transfer Control (ETC)." McDonnell Aircraft

Company, NASA CR-I14541, June 1972.

12. Anon, "Advanced Energy Transfer Control Ducting

Investigation". McDonnell Aircraft Company, NASA CR-I14539,

Aug. 1972.

13. Deckert, W.H. and Evans, R.C., "NASA Lift Fan V/STOL

Transport Technology Status." Paper 720856, Society of

Automotive Engineers, National Aerospace Meeting, Oct. 1972.

32

14. Anon, "Conceptual Design of a V/STOL Lift Fan

Commercial Short Haul Transport." McDonnell Aircraft

Company, NASA CR-2184, Jan. 1973.

15. Knight, R.G.; Powell, W.V. Jr.; and Prizlow, J.S.,

"Conceptual Design Study of a V/STOL Lift Fan Commercial

Short Haul Transport." North American Rockwell, NASA CR-

2185, April 1973.

16. Eldridge, W.M.; Ferrell, J.A.; McKee, J.W.; Wayne,

J.F. Jr.; and Zabinsky, J.M., "Conceptual Design Studies of

Candidate V/STOL Lift Fan Commercial Short Haul Transport."

The Boeing Company, NASA CR 2183, Feb. 1973.

17. Deckert, W.H. and Roils, L.S., "Integrated

Propulsion/Energy Transfer Control Systems for Lift-Fan

V/STOL Aircraft." Paper 135, AGARD Propulsion and

Energetics Panel Meeting on V/STOL Propulsion Systems,

Sept. 1973.

18. Deckert, W.H. and Holzhauser, C.A., "Evaluation of

V/STOL Research Aircraft Designs." Paper 730947, Society of

Automotive Engineers, National Aerospace Engineering and

Manufacturing Meeting, Los Angeles, CA, Oct. 1973.

19. Anon, "Conceptual Design Studies of a V/STOL Civil

Lift Fan Transport Including Effect of Size and Fan Pressure

Ratio." McDonnell Aircraft Company, NASA CR-2426, April

1974.

20. Anon, "Conceptual Design Studies of a V/STOL Military--

Civil Lift Fan Aircraft." McDonnell Aircraft Company, NASA

CR-137486, May 1974.

21. Cavage, R.L., "Conceptual Design Study of a Military

Mission Oriented Remote Lift-Fan V/STOL Transport." Rock-

well International Corporation, NASA CR 114753, July 1974.

22. Anon, "Design Definition Study of a Lift/Cruise Fan

Technology V/STOL Aircraft, Volume I, Navy Operational

Aircraft." McDonnell Aircraft Company, NASA CR-137678, June1975.

23. Anon, "Design Definition Study of a Lift/ Cruise Fan

Technology V/STOL Aircraft, Volume II, Technology Aircraft."

McDonnell Aircraft Company, NASA CR-137698, June 1975.

24. Zabinsky, J.M. and Burnham, R.W., "Design of a V/STOL

Technology Airplane." The Boeing Company, AIAA Paper No. 75-

1012, August 1975.

25. Zabinsky, J.M. and Higgins, H.C., "Design Definition

Study of a Lift/Cruise Fan/Technology V/STOL Airplane --

33

Summary." The Boeing Company, NASA CR-137749, Aug. 1975.

26. Cavage, R.L., "Design Definition Study of NASA/Navy

Lift/Cruise Fan V/STOL Aircraft, Volume I -- Summary Report

of Navy Multimission Aircraft." Rockwell International

Corporation, NASA CR 137695, July 1975.

27. Cavage, R.L., "Design Definition Study of NASA/Navy

Lift/Cruise Fan V/STOL Aircraft, Volume II -- Summary Report

of Technology Aircraft." Rockwell International Corpora-

tion, NASA CR 137696, June 1975.

28. Rolls, L.S., "Operational Experience with Gas-Driven

Remote Lift/Cruise Fans in V/STOL Applications." NASA

Working Paper, 1975.

29. O'Brien, W.J., "Lift/Cruise Fan V/STOL Technology

Aircraft Design Definition Study, Volume I, Technology

Flight Vehicle Definition." McDonnell Aircraft Company,

NASA CR-151931, Nov. 1976.

30. Anon, ............ Studies for Design Definition of a

Lift/Cruise Fan Technology V/STOL Airplane, Volume I,

Technical Report." The Boeing Company, NASA CR-137976,

Jan. 1977.

31. Deckert, W.H., "Recent V/STOL Aircraft Designs."

Journal of the American Helicopter Society, Volume 24-

Special, June 1979.

32. Roberts, L.; Deckert, W.; and Hickey, D., "Recent

Progress in V/STOL Aircraft Technology." NASA TM-81281,March 1981.

33. Roberts, L. and Deckert, W. H., "Recent Progress in

V/STOL Technology." The Aeronautical Journal of the Royal

Aeronautical Society, Paper No. 1013, Oct. 1982.

34. Deckert, W.H. and Franklin, J.A., "Powered-Lift

Technology on the Threshold." Aerospace America, Nov. 1985.

35. Deckert, W.H. and Franklin, J.A., "Powered-Lift

Aircraft Technology." NASA SP-501, 1989.

34

90K

I00

8o

Go

C,. 40

o

20l.=.J

0

-----_._-.- /V = 60 knots

:-..

Qts

J I , ! I

2000 4000

AIRFIELD LENGTH, feel

Figure 1. Percent of existing fields which can be used

by aircraft with different approach speeds and field

length requirements

90K

f 60K

60K

AIRFIELD

I J

2000 feel

Figure 2. Effect of approach speed on pattern size

35

- CEILING AT 90K

iI "_1__ ILING AT 6OK

VISIBILITY AT 90K 60K

Figure 3. Benefits of reduced approach speed under WR conditions

I00 -

E

u 80-

0C_

@1

Q)U

Uo

60 L

60-

60-

"=._ JET

D • I00 slolule md_

TURBO PROP

I I I I

TURBO_

D, 300 slolute m_les

40 t I , I i20 40 60 80 IO0

APPROACH SPEED, knols

Figure 4. Direct operating cost for aircraft with

different low-speexi characteristics

36

TILT ROTOR

/-,,

e t

LIFT-FAN

STOWED ROTOR

," °. .... "_-_'_ ' °t,..o

TILT WING

o,

LIFT JET

Figure 5. VTOL short-haul transport concepts studied

37

Estimated C.G. location

at D.G.W,

Fuel in wings /X //J I-,,-MAX 195'

I _''_-:':_... _ "--- 'l '- : "'-,-- - '" APU1 ,,,., , i ____t\' -- - _, t .... - : - \ ....... _ ..........

Airstai,',,_ : Caroo i CarQo \ Air,,tairs-j _ ,,-- 31.4' -_J (under floor) _1

82.5'

33.5

Figure 6. Boeing 60-passenger VTOL lift-fan aircraft design

38

OUTER GAS GENERATORS FOR TAKEOFF

GAS GENERATOR FOR TAKEOFF & CRUISE

CRU LIFT F_

SECTION THROUGH CENTER ENGINE

Figure 7. Lockheed 60-passenger VTOL lift-fan aircraft design

39

120

u)¢_

O_r-

u)

Q.M,,-

O

JDE

z

90

Rotor or prop

60 J100 200 300 400 500

Stage length (mile)

600

Figure 8. Summary of promising missions for VTOL designs in terms of stagelength and payload

DEFLECTED SL IPSTREAM TURBOFt, N

LIFT FAN PROPULSIVE WING

Figure 9. STOL short-haul transport concepts studied

4O

f

Figure 10. Vought 60-passenger STOL lift-fan propulsive wing

aircraft design

41

', -:4_,',! #' °' t..'.!lI

ill_ I "lllli

I I

I I

_tt I II ,'.'I

;iil,, ",lit,, t _

I

!

D!I

!

• I I

• I !

i I I

• 1 I• , |

# . I

s # i |

• i !

,II

!

!

II

I

II

!

1

1

i

I

I

I

I I

11

'i

/z//}////

lil

,; '

,/R/R/_/,bill! I #t_ Ii t_i_ i, t4_ li ._ll I III

[fiN', ; i IN_/_ i l i 1 I I

L _ /

! ,I

!

I

r

o jI

, I

m,I

I

! •%

I •

i %%

I •

I •

I •

_l '%.

Figure 11. Vought propulsive wing propulsion system planform

42

_FAN FAN TURBINE 7

HOT GA_///_ //TURBINEDUCT EXHAUST

NOZZLE' . Y ,,,

i ....................FAN'::;::<_-XHAU ST

SFAN INLET NOZZLE

Figure 12. Vought propulsive wing cross section

Figure 13. McDonnell modified DC-9/STOL lift fan plus lift/cruise

fan research transport design

43

i

o

Figure 14. Boeing modified BuffaloV/STOL liftfan pluslift/cruisefan research

transport design

"_-- 320'" ;..IOAL_ ___8

645" _ m:

SgO'" _ /

t! I,I

,.C_ "_ _ --_ --3' _°

Figure 15. Boeing 100-passenger WSTOL integral lift-fan transport dcsign

44

Figure 16. McDonnell 100-passengerV/STOLremotelift-fantransportdesign

C r-----1

CaDUCT INTERCONNECT

SCHEMATIC

Figure 17. McDonnell propulsion schematic for design with six gas generators/six fans

45

Emergency fan out(inboard of Nacelle)

Gas generator

Gas generator

Emergency fanout nozzle

Lift fan

Lift/cruise fan

Figure 18. McDonnell 100-passenger V/STOL transport design

0ater version)

FWD I

FANOUT NOZZLE/-¢ (TYP 4 PLACES}

VALVES

AS GENERATOR ISOLATION ..... 4

ETaC MOD. + SHUTOFF .......... 12

-7/-- SYSTEM ISOLATION ............. 2

TOTAL 18

Figure 19. McDonnell propulsion schematic for design with four

gas generators/four fans

46

Figure20. Rockwell 100-passenger WSTOL remote lift-fan transport design

47

____1_ CON']'ROL &

_ v/ /"7' SHUTOFF

//

.___ D IVERTER

VALVEEMERGENCY

NOZZLE

CONTROL VALVE

IX]

\SHUTOFF VALVE

FOR STARTING

INTERCONNECT DUCT

i:_1"_----..-_ BACK_FLOw

VALVE

Figure 21. One of four paired gas interconnected systems in the Rockwell

100-passenger design

4,5,8 _.. " -'

SEPARATE HOT GAS DUCT SYSTEM

GAS GENERATOR FAILURE

GAS GEN ACTION RESULT IS HALF

FAILS TAKEN THRUST FOR FANS

F REPLACESA OR B A OR B 5 & 6

H REPLACESC OR D CORD 7 & 8

E OR F NONE 5 & 6

C OR H NONE 7 & 8

FANFAI LS

1OR2

3 OR 4 4 OR 3

5 OR 6i 6 OR 5

7OR 8] 8 OR 7

FAN FAILURE

SHUT DOWN DIVERT GAS TO EMERGFAN FROM GAS GEN NOZZLE

2OR1 A&B 1&2

C&D 3&4

E&F

G&H

Figure 22. Propulsion system schematic for the Rockwell 100-passenger design

48

Lift/cruise fan

Gas generator

Gas generator

Interconnect duct L

Lift fan

Figure 23. McDonnell STOVL design for Navy Vcrtical-Onboard-Delivcry

_,L

®

VALVES

GAS GEN. ISOLATION = 3

/

SYSTEM ISOLATION = 2

ETaC VALVES = 6

TOTA L = 1 1

Figure 24. Propulsion schematic for McDonnell VOD design

49

0.467 SCALE

Jl0l TECH GAS

GENERATOR

ISOLATIONVALVE

LIGHTWEIGHT CROSS DUCTING

VALVES

1.5 FPR 2-STAGE

VTO DESIGN FANS

QUAD ENTRIES

CONTROL VALVESINTEGRATEDSINGLESWIVEL NOZZLES

Figure 25. Rockwell STOVL design for Navy Vertical-Onboard-Delivery

//[ _ J / Interconnect ducting _ _Cruise thrust

W _ k,.._ C / \__ \\_'_ Swivel nozzle

Gas gen_erator _7 Ti 'pturbine _--_ Lift thrust

driven fan

Figure 26. Single swivel nozzle on lift/cruise fan on Rockwell VOD design

50 -.

1.5 FPR, vro /,\

F 73.99 IN. "1TWO - STAGE

_._, _o _______

86.26 IN.

SINGLESTAG[

Figure 27. Two-stage fan on Rockwell VOD design, with comparison

I72.73

l

DUAL ENTRY

i

Iq 73.99

1.5 FPR, VTO

166 IN.

l

QUAD ENTRY

I

lq 69.64 IN. .

Figure 28. Quad entry scroll on Rockwell VOD design, with comparison

51

Figure29. BoeingV/STOL designfor Navy multimissions

Clutch

Cross shaft

T-box

• Accessories

Reduction gear

Lift/cruise fan

Lift fan

Drive shaft Shaft support

Bevel gears (h/p)

Figure 30. Isometric of Boeing Navy multimission design

52

Lift fan

_k_ BeveF gear box

..... --

Drive shaft

Friction and

jaw clutch

. _.--_

//

//

Cruise/lift fan

---- T-box

Bevel gears

AGB

Sym about _.

- Clutch-sprang

Engine

Bevel gears _ / k._._ Star gearing

Figure 31. Power train schematic of Boeing Navy multimission design

,-- Variable-pitch fan

Cruise position ;haft

Blow-in-door TeL Overrunning clutch

!

• Variable area nozzle

Cruise and loiter position

Yaw controt varies

Allison T701

J

Reduction gear and Mounting struts and

variabte-p_tch mechanism Straight vanes

Engine accessones

Figure 32. Integral lift/cruise fan pod on Boeing Navy multimission design

53

Engine pod

rotatingsupport tube

Bearing block! interconnect chord

Bearing race /block (4 places) / Drive,

Shaft coupler

Tee box

/f

Engine

supportring

i;.

/

_ri_e

Splfl half

Drive molors

Wire and hydraulic __i

service lines Io engines _./

i Drive shaft to forward tan

r- - Main support frames

Figure 33. Pivot mechanism for lift/cruise fans on Boeing Navy multimission design

54

-- Main fan drive gear box_- Fwd fan drive shaft _...-_-Space for shaft-driven

airplane accessories and access

__._ E- .... .,P

I -\ ¢L;¥-J_=._'o_.f.,ii II \ t{'_--7--[.....T

--........ _--_ -B_ r_g"- - _ "_ Disconnect clutch

_--" _-- Coupling__.J__ ............. L _ ......

Figure 34. Nose lift-fan shaft drive system on Boeing Navy multimission design

-._<,.....:._,... ..............._7.:J:_'i,:. :_.. _.-_.-:_._.,.

•.,.,, .... ._,_'A, " . • -. - .'--".......:-: :."=:"_"_": ".... _ "_:_' ,.. '"_,";._4' " """ :'_ . " "

..g,.-_;,-. ._:._7" ",. • ; - _ : .': ; :_: -..

,::-.,.,_i". .... . ' ";.,'#, ,-,'_" .'+"'_'. ..... " ' ."" "_" '"_'*-

... ,_.._.-::..................... _--._j_-_ _.,_.-_

;¢.,.,.,.,.,.,.,.,._<.... ."',,_z"_ __-'-._ :"+-.

'._ :_-.,.<:.. ;:,.....:...'7... ,,:. _..7-.;.117"I

Figure 35. McDonnell V/STOL design for Navy multimissions

55

Fuel capacity

Aft fuselageFwd fuselageWing

1200 ib2300 Ib

8200 Ib --_

\

In-flight refueling probe ///

Jet fuel starter "/_

hydraulic pump Jand generator

/

\

| II

Hydraulic pumpand generatoreach L/C fan

Accomodations for crew of 4• " ats

Figure 36. General arrangement for McDonnell Navy multimission design

56

\\

Figure 37. Rockwell V/STOL design for Navy muldmissions

ETaC

VALVES

59 IN. TURBOTIP FAN

3rd ENGINEDIVERTER

VALVE

J-97 GAS

GENERATOR --_ HOOD

LIFT FANINL_

DUCTING

LI FT FANVECTORING SYSTEM

GAS GENERATORINLET

Figure 38. McDonnell modified T-39 RTA V/STOL aircraft design

57

(A) DIVERTERVAL

LIFT/CRUISE

ETaC MODULATIONVALVE

113 SCROLL

SHUTOFFVALVE

(Ci NOSE FAN ETaCAND SHUTO

:) ENGINE ISOLATIONVALVE

(C] INTERCONNECTISOLATION VALVE

(B) LIFT/CRUISE ETa _C_

MODULATIONVAL VE

(C) BUTTERFL_

VAL VE

Figure 39. Propulsion/controland valving schematic forMcDonnell T-39

RTA design

58

ETaCMODU LATIONVALVE

1/3 SCROLLSHUTOFF VALVE

Figure 40. Scroll-In-Scroll on the McDonnell T-39 RTA design

Thrul! veclortng and reductionmodulation louvers

land closure doorsl

Figure 41. Thrust ReductionModulation louvers and vanes in

nose tan exit, McDonnell T-39

RTA design

59

Thrust reduction modulation(TRM) doors Rotating hood

segments forthrust vectoring

Yaw control vanesand closure doors

Figure 42. Thrust Reduction Modulation, hood, and vanes on lift/cruise fan,

McDonnell 1"-39 RTA design

Figure 43. Boeing modified T-39 RTA V/STOL aircraft design

6O

AR • 5.77, S, - _12 FT2..A.,_• 32.5"

YV __ L r

DBL SLOT

44.43FT

_ w_ 18.5sFT

Figure 44. Rockwell modified T-39 RTA V/STOL aircraft design

61

(3) BASIC J97 GE 100 _(2) LCF450 WITH 10'I, TURBINE BLOCKAGE I /

_-_-_:C_RCULAR SCROLL DES'GN /'_-_.._

/ (_ _ / olNTEGR SWIVEL NOZZLE

!. _N_ '_/JJ "LIDGuHcTT_E,GHTHOT GAS

Figure 45. Propulsion system isometric, Rockwell T-39 RTA design

Figure 46. Large-scale model of Grumman V/STOL twin tilt nacelle design

62

NACELLE PIVOT LINE

BL0

CARRY THROUGH_

STRUCTURE._.:

TF-34-GE-100

TURBOFAN

VANE SUPPORT BOOM

HORIZONTALVANE AND FLAP

TOP VIEW

NACELLE PIVOT_ VERTICAL

AND _ VANEBULLET

SUPPORTVANES _

SIDE VIEW

Figure 47. Propulsion schematic of Grumman twin tilt nacelle

design

Figure 48. McDonnell V/STOL twin fixed nacelle design

63

tROTATING

I

VENTING LIP _ _'_S_IPLIT YAW

VANE

DIFFERENTIALLYVECTORED

POWER _ "_ THRU_;,

TRA::::R/______ ___JYAW

Figure 49. Propulsion/control schematic of

McDonnell twin nacelle design

Y J-97

Figure 50. Full-scale investigation of characteristics of manifolded YJ-97 gas generators

64

ENGINE NO.I ENGINE NO. 2

ISOLATION VA[

/ETC VALVES AT

ETC SHUTOFF VALVES

NOZZLES NO.I NOZZLES NO. 2

Figure 51. Initial configuration for investigating transient behavior

during failure of no. 2 gas generator

9°E.VALVE ANGLE, 60

degrees 300

ENGINE SPEED,

percent

PRESSURERATIO

NOZZLETEMPERATURE

RATIO

-- ISOLATION

--- No., //.. _;J------ NO. 2

i 1 J-.t-", I I

5O "_-

0" -w

I 1 I l I

I..----ITH ROTTLE ..I----I.O [-_\ NO. 2 ENGINE /I

.8I- 1_,"_CLOSED _----" _.---

.9_._.L\ _ --/ --__._

.70 I 2' 3 4 5 6 7

TIME, seconds

Figure 52. Time history of simulated failure of no. 2 gas generator

65

Gas

Flow

Inner

Liner

Insulation

Pressu re

Vessel

Figure 53. Composite duct concept

Figure 54. Full-scale duct wire wrapped screen liner

66

.............. c_

Figure 55. Final two components ready for assembly into the full-scale

composite duct segment

1600 -

i,0

I.Srr-

I,--

rr"I.iJ0,.

LdI--

1200

8OO

4OO

\GAS TEMPERATURE

\\\

?TEMPERATURE OF INNER

WALL OF PRESSURE DUCT

I 1 I l

0 50 I00 150 200

TIME, sec

I

250

Figure 56. Experimental one engine-out temperature time history for the composite

duct segment

67

Isolation Valve No. 2--_

Interconnect Duct---_ Shutoff Valve _ /

, J .ETC Valves No. 2 J

_ i_! I _=__EngineNo. 1

_--Isolation Valve No. 1

ETC Valves No. 1

Fan No. 1

Figure 57. General arrangement of a paired Energy Transfer Control system

68

t

Figure 58. Full-scale ETC valves

69

Figure 59. Full-scale interconnect duct shutoff valve

ENGINE

NO. I

_15% OF ENGINE NO. I GAS FLOW

ENGINE

NO. 2

m

i

=,..am

/ sj --ETC VALVE AT 40 ° AND O"

NOZZLE DISCHARGE RATIOS

1.21 TEMPERATURE/TEMPERATURE (NO CONTROL) 1.20

.84 PRESSURE/PRESSURE (NO CONTROL) I.i 3

.85 GAS FLOW/GAS FLOW (NO CONTROL) 1.15

.99 GAS ENERGY/GAS ENERGY (NO CONTROL) 1.64

Figure 60. Steady-state gas conditions at lift-fan scroll entry with

one pair of ETC valves deflected 40 deg and both YJ-97s operating

at rated speed

7O

ETC

VALVE

ANGLE,

deg

ENGI I

NO. NE

/'LB-': _ ETC/_ " \ AT(! ' /\,, . , //

4O

50

20-

I0-

0-6000

VA LV E

40 °

F_ ENGINE

/ -I NO. 2

(, ....._I\, ;i

\ .._._"

""-, NO. I FAN_ "",, F"

>.NO. I FAN WITH / \ E\

SPOILAGE \k.\

f

i ] { i-4000 -2000 0 2000

INCREMENTAL FAN THRUST,

NO. 2 FAN

I !

4000 6000Ib

Figure 61. Calculated incremental lift-fan thrust based on measured gas energy

for no. l ETC valves deflected from 0 to 40 deg

ENGINE NO. I ENGINE NO. 2

NOZZLES NO.I

ISOLATION VALVE / .

ETC VALVES AT O*_._____1

ETC SHUTOFF VALVES _ L_ J

NOZZLES NO. 2

Figure 62. Initial configuration for investigating system behavior with

no. 2 YJ-97 inoperative

71

INCREMENTALFAN

THRUST,

newtons

1.6X o4

DIFFERENTIAL FOR CONTROL

1.2

.8

.4

0 i I !

-.B I I I I t 1 I ,!

0 .2 .4 .6 .8 1.0 1.2 1.4TIME AFTER ETC VALVE NO. I COMMAND, seconds

Time history of lift-fan incremental thrust after a 30 deg stepPigure 63.

input of ETC valves no. 1, with no. 1 YJ-97 at 97 percent speed and no. 2

YJ-97 inoperative

Figaare 64. USA VZ-9AV/Avro Aircraft Limited Avrocar hovering 1 foot above concrete

....... 72

Observer's cab

\d 69.T.9 turbojet

Air intake Turborotor assembly i Rear cargo trunkI

Engine intake

Fueltank Operator's cab

Figure 65. Artistic schematic of the Avrocar

73

\\

/Springs now located here

fFocussing ring

Focussing ring control ]

Spoiler control _

Spring location

Figure 66. Avrocar original spoiler control system and final focusing ring control system

74

moved aft

neutral __._.!_:; . !,:.!_:,".:

:_ii!! i.i:i../:_i:_i_•

Figure 67. Effect of the focusing ring on the lift-fan annular nozzle jet exhaust

75

YAW VANES

COLLECTIVEFOR

IN FLIGHT

\TRANSITION DOORS

ELECTRICSCREW JACK

MODIFIED HANGER RODS

CONTROL

COUPLING

THIS SECTORUNCHANGED

BETWEENHOVERING

AND FORWARD FLIGHT

HOVERING CONTROL

( FOCUSSINGRING )

FWD

IN FLIGHT PITCH AND ROLL

CASCADES CONTROLVANE

Figure 68. Illustration of design for transitioning from the low-speed to the

high-speed control system

A.

B°

Co

f

critical height

.1

-T Flow belowcritical height

C.P,

Flow just belowcritical height

L_ Flow above

critical height

Figure 69. Illustration of cause of ground cushion

instability at the critical hovering height

76

Form Approved

REPORT DOCUMENTATION PAGE OMSNoo7o4-o 88Public reporting burden for this collectionof informationis estimated to average 1 hourper response, includingthe time for reviewing instructions,searching existing data sources,gatheringand maintaining the data needed, and completingand reviewing the collectionof information. Sendcomments regardingthis burden estimate or any other aspect of thiscollectionof information, includingsuggestions for reducingthis burden, to WashingtonHeadquarters Services, Directoratefor informationOperations and Reports, 1215 JeffersonDavis Highway, Suite 1204, Arlington,VA 22202-4302, and to the Office of Management and Budget,Paperwork ReductionProject (0704-0188), Washington, DC 20503.

1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE 3. REPORT TYPE AND DATES COVERED

September 1993 _ Contractor Report4. TITLE AND SUBTITLE 5. FUNDING NUMBERS

The Lift-Fan Powered-Lift Aircraft Concept: Lessons Learned

6. AUTHOR(S)

Wallace H. Deckert

7. PERFORMINGORGANIZATIONNAME(S)ANDADDRESS(ES)

NASA Ames Research Center

STOVL/Powered-Lift Technology Branch

Moffett Field, CA 94035-1000

9. SPONSORING/MONITORINGAGENCYNAME(S)AND ADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546-0001

A25364D

8. PERFORMING ORGANIZATION

REPORT NUMBER

A-93107

10. SPONSORING/MONITORINGAGENCY REPORT NUMBER

NASA CR- 177616

11. SUPPLEMENTARY NOTES

Point ofContact: K. ClarkWhite, Ames Research Cente_MS 237-3, Moffett Field, CA94035-1000;

(415)604-5653

12a. DISTRIBUTION/AVAILABILITY STATEMENT

Unclassified -- Unlimited

Subject Category 05

12b. DISTRIBUTION CODE

13. ABSTRACT (Maximum 200 words)

This is one of a series of reports on the lessons learned from past research related to lift-fan aircraft concepts. An

extensive review is presented of the many lift-fan aircraft design studies conducted by both government and industry

over the past 45 years. This report addresses mission applications and design integration including discussions on

manifolding hot gas generators, hot gas dusting, and energy transfer control. Past lift-fan evaluations of the Avrocar

are discussed. Lessons learned from these past efforts are identified.

14. SUBJECTTERMS

Lift-fan, Powered-lift, Short takeoff/vertical landing (STOVL)

17. SECURITY CLASSIFICATION

OF REPORT

Unclassified

18. SECURITY CLASSIFICATIONOF THIS PAGE

Unclassified

19. SECURITY CLASSIFICATIONOF ABSTRACT

15. NUMBER OF PAGES

8O16. PRICE CODE

A06

20. LIMITATION OF ABSTRACT

NSN 7540-01-280-5500 Standard Form 298 (Rev. 2-89)Prescribed by ANSI Std. Z39-18